The present invention relates generally to measurement of distance or ranging by exploitation of near-field electromagnetic behavior and especially to a system and method for evaluating a distance between a transmitter or beacon and a receiver or locator. Still more specifically, the present invention describes a means for determining a range to a transmit-only beacon without requiring synchronization, and without relying on variation in signal amplitude. The present invention may be advantageously used as part of a more general system for determining position (range and bearing) or for tracking (determining position in near real time).
Related Art
A variety of techniques are known in the art for using electromagnetic signals to determine direction and distance. These techniques are sometimes referred to as radio direction finding and radio ranging. A good summary of the state of the art in radio direction finding is provided by Jenkins. [Small-Aperture Radio Direction-Finding, by Herndon H. Jenkins; Artech House, Boston; 1991; pp. 1-23.]
Time Difference and Phase Difference Angle of Arrival
One technique for radio direction finding has been dubbed time difference of arrival (TDOA). This technique uses a pair of co-polarized antennas separated by a baseline distance. An incoming signal incident in a direction perpendicular to the baseline is received by both antennas at the same time. When the direction of incidence is not perpendicular to the baseline, one antenna will receive the signal before the other. The difference in the time of arrival of the signals at each antenna can be related to the angle of incidence. Equivalently, this difference in time of arrival may be treated in a manner similar to phase difference. Using this technique, the direction of arrival of an incident plane wave may be determined. This TDOA technique may be generalized to apply to a network of receiving antennas at known positions. By comparing the times of arrival of the signal at each receiving antenna, the direction of the incident plane wave may be determined. In many (but not necessarily all) circumstances, the direction from which the incoming plane wave arrives is the direction in which a target transmitter resides. Early examples of such radio direction finding systems include the direction finding systems disclosed by J. S. Stone (U.S. Pat. No. 716,134; U.S. Pat. No. 716,135; U.S. Pat. No. 899,272; U.S. Pat. No. 961,265) and Roos (U.S. Pat. No. 984,108). Phase detection for angle of arrival in the manner now generally understood in the current art was disclosed by Fritz (U.S. Pat. No. 2,160,135) by Runge (U.S. Pat. No. 2,234,654) and by Budenbom (U.S. Pat. No. 2,423,437). 3-D radio direction finding using phase difference was disclosed by Jansky (U.S. Pat. No. 2,437,695). Lioio et al (U.S. Pat. No. 5,724,047) disclose a phase and time difference radio direction finding system.
Antenna Pattern Angle of Arrival
Another technique for radio direction finding involves using an antenna whose response varies as a function of angle. In one implementation, one might use a directive antenna with a relatively narrow beam-width in a particular boresight or direction of maximum signal strength. The orientation of the antenna is varied until the received signal is maximized so that the boresight of the antenna is aligned with the direction of the incoming signal. In an alternate implementation, one might use an antenna with a null in a particular null direction or direction of minimum signal strength.
In one early invention, Erskine-Murray et al (U.S. Pat. No. 1,342,257) disclose the use of a loop antenna rotated about an axis lying in the plane of the loop. A similar apparatus that allowed finding a minimum or null while still receiving a signal was disclosed by Robinson (U.S. Pat. No. 1,357,210). Two loop antennas with orthogonal axes may be electrically combined so as to create a virtual antenna oriented in the direction of a signal maximum (or minimum). A capacitive combining arrangement or goniometer was disclosed by Bellini (U.S. Pat. No. 1,297,313), and a transformer or inductive coupling goniometer was disclosed by Goldschmidt et al (U.S. Pat. No. 1,717,679). An electrically small loop and an electrically small whip (or dipole) antenna may be combined to yield a cardiod type pattern with a sharp null in a particular azimuthal direction. The orientation of the antenna may be varied until the received signal is minimized, then the null direction is aligned with the direction of the incoming signal. Examples of this technique are disclosed by Taylor (U.S. Pat. No. 1,991,473), Bailey (U.S. Pat. No. 1,839,290), and Busignies (U.S. Pat. No. 1,741,282). The technique of goniometer combination of signals from directive antennas was also disclosed by Fischer (U.S. Pat. No. 2,539,413).
Amplitude Comparison Angle of Arrival
Still another technique for determining the angle of arrival of a radio wave is amplitude comparison angle of arrival. The signal amplitudes of two or more antennas are compared so as to determine angle of arrival. For instance if a first antenna signal amplitude is very large and a second antenna signal amplitude is small, one can infer that the radio wave arrived from the direction of the first antenna's pattern maximum and the second antenna's pattern minimum. If the signals are of comparable size, then the radio wave may have arrived from a direction in which the two antennas' patterns have comparable directivity. This is similar to the traditional goniometer angle of arrival technique already mentioned. Examples of this technique include disclosures by Earp (U.S. Pat. No. 2,213,273), Wagstaffe (U.S. Pat. No. 2,213,874), Budenbom (U.S. Pat. No. 2,234,587), and Clark (U.S. Pat. No. 2,524,768).
Doppler Angle of Arrival
Yet another technique for radio direction finding takes advantage of the Doppler-Fizeau effect. If a receive antenna is rotated at high speed about an axis perpendicular to the direction of an incoming signal, then that incoming signal will be shifted up in frequency as the receive antenna moves toward the direction of the incoming signal and down in frequency as the receive antenna moves away from the direction of the incoming signal. In practice, it is not feasible to rotate an antenna at a high enough angular velocity for this effect to be readily observable. Instead, a number of receive antennas may be placed in a circle and sequentially scanned or sampled at a high rate in order to simulate rotation. Such systems were disclosed by Earp (U.S. Pat. No. 2,651,774) and Steiner (U.S. Pat. No. 3,025,522).
Hybrid Angle of Arrival
The prior art techniques discussed hereinabove for making angle of arrival measurements may be advantageously combined. For example, Edwards et al (U.S. Pat. No. 2,419,946) disclose the combination of amplitude and phase comparison in a radio direction finding system. Murphy et al (U.S. Pat. No. 5,541,608) disclose combining amplitude and phase comparison in a radio direction finding system. Murphy et al do not employ their disclosed architecture to measure range or distance, and they do not employ near-field behavior of electromagnetic signaling as is taught by the present invention.
Triangulation
A variety of radio direction finding measurements from a network of two or more dispersed positions allows the location of a target transmitter to be determined. One technique by which this can be accomplished employs triangulation. For example, if the direction to a target transmitter has been determined from three known positions, the bearings for the three directions may be plotted on a map, and the location of the target transmitter is at the intersection of the bearings, or by the triangular region bounded by the intersections of the bearings. An example of such a system was disclosed by Maloney et al (U.S. Pat. No. 4,728,959).
Radio Ranging
Radio ranging may be accomplished by triangulation from a collection of direction finding measurements. However, a disadvantage to this prior art ranging technique is that obtaining even a single range or distance calculation requires measurements taken from at least two different positions. The positions must be separated by a baseline that is a significant fraction of the range to be measured in order to obtain a reliable range determination.
RADAR
There are a variety of other ways in which range may be measured. One technique is RAdio Detection And Ranging (RADAR) such as is disclosed by Plaistowe (U.S. Pat. No. 2,207,267). The radar technique relies on the scattering of signals from a target. Radar works well in the detection of aircraft in an open sky or ships on the surface of an ocean but radar detection becomes increasingly difficult when the target being tracked is in a cluttered environment populated by scatterers of equivalent cross-section to the target one desires to track.
Passive Tag Ranging
A passive cooperative target, passive transponder, or passive tag yields better performance than is achieved with an uncooperative radar target. In a passive tag ranging system, a transmitter radiates a signal that is received by a passive transponder. The passive transponder takes the received energy and reradiates the signal. The reradiated signal is received at the original transmitter and compared to the original transmitted signal. This comparison may involve phase, time delay, or other comparison between the transmitted and received signals which enables a range measurement. An example of such a system is disclosed by Lichtenberg et al (U.S. Pat. No. 4,757,315). A disadvantage of passive tag ranging is that the effective range tends to be relatively short due to the low power picked up by the tag that is available to be reradiated.
Active Transponder Ranging
An active cooperative target is generally more effective in ranging operations than a passive target. An active transponder listens for a particular interrogatory signal and responds with a particular reply signal. The frequency of the reply signal is not necessarily the same as the interrogatory signal, and the strength of the return signal is not dependent on the strength of the interrogatory signal received by the target. This technique may be referred to as active transponder ranging. The time of flight from an interrogating transmitter to the transponder and back to a receiver may be determined by a phase comparison of the original transmitted signal with the signal received from the remote transponder. In some embodiments, the phase comparison may be performed on a modulation imposed on an interrogatory signal and a reply signal. Knowing the wave velocity of signals, the time of flight may be translated into a distance. Examples of transponder type ranging systems include disclosures by Green (U.S. Pat. No. 1,750,668), Nicolson (U.S. Pat. No. 1,945,952), Gunn (U.S. Pat. No. 2,134,716), Holmes (U.S. Pat. No. 2,198,113), and Strobel (U.S. Pat. No. 2,248,727). Deloraine et al (U.S. Pat. No. 2,408,048) disclose a system for using time modulated pulses in a transponder ranging system. Nosker (U.S. Pat. No. 2,470,787) discloses a system for 3-D position measurement using transponder ranging, and Williams (U.S. Pat. No. 3,243,812) discloses a particularly simple transponder system involving cycle counting of a phase comparison between a transmitted signal and a received transponded signal. A disadvantage of transponder ranging is that it requires an active target to receive a signal, and transmission of a return signal is generally influenced by some property of the received interrogatory signal.
Transmit-Only Ranging
A simpler transmit-only ranging scheme uses a transmit-only target. One way to implement a transmit-only ranging system is to measure the amplitude of signals received from a transmitter of known transmit power. This amplitude ranging method of radio ranging was disclosed by de Forest (U.S. Pat. No. 749,436; U.S. Pat. No. 758,517; U.S. Pat. No. 1,183,802). In some cases the amplitude decreases in a predictable fashion with distance from the receiver. For instance in free space, received power varies as the inverse square of the distance. Knowing the transmit power, the receive power and the properties of the antennas, one can infer the range using a known relationship, such as Friis Law.
The relationship between transmitted power (PTX) and received power (PRX) in a far-field RF link is given by Friis Law:                               P          RX                =                              P            TX                    ⁢                                           ⁢                                                    G                TX                            ⁢                              G                RX                            ⁢                              λ                2                                                    4              ⁢                                                           ⁢                              π                2                            ⁢                              r                2                                                                        [        1        ]            
where GTX is the transmit antenna gain,                GRX is the receive antenna gain,        λ is the RF wavelength, and        r is the range between the transmitter and receiver.        
Power rolls off (i.e., power decreases as range increases) in the far-field as the inverse square of the distance       (          1              r        2              )    .Near-field links do not obey this relationship. Near-field power rolls off at powers higher than inverse square, typically inverse fourth   (      1          r      4        )or higher.
This near-field behavior has several important consequences. First, the available power in a near-field link tends to be much higher than would be predicted from the usual far-field, Friis Law relationship. This results in a higher signal-to-noise ratio (SNR) and a better performing link. Second, because the near-fields have such a relatively rapid power roll-off, range tends to be relatively finite and limited. Thus, a near-field system is less likely to interfere with another RF system operating outside the operational range of the near-field system.
Inferring range from received signal power or amplitude is problematic at best. Despite the difficulties, amplitude ranging systems are still used. For instance Moulin (U.S. Pat. No. 5,955,982) disclosed a method and device for detecting and locating people buried under an avalanche in which signal amplitude is used to localize an avalanche victim.
There are a variety of other ways by which a receiver can obtain range information from a transmit-only target. Ranger (U.S. Pat. No. 1,639,667) disclosed the idea of synchronized oscillators at a transmitter and at a remote receiver. A receiver can compare the number of 360° phase shifts or the number of beats per time to infer a change in distance. In a series of inventions, Gage (U.S. Pat. No. 1,828,531; U.S. Pat. No. 1,939,685; U.S. Pat. No. 1,939,686; U.S. Pat. No. 1,961,757) disclosed transmitting a pair of signals at different frequencies with different propagation characteristics and different attenuation constants. By comparing the amplitude ratio of the received signals, range may be inferred. Runge (U.S. Pat. No. 2,134,535) disclosed looking at the superposition of direct and reflected rays in a received signal to infer range from a transmitter. Herson (U.S. Pat. No. 2,314,883) disclosed evaluating the rate of change of the amplitude of a received signal in order to infer range. Hammerquist (U.S. Pat. No. 4,788,548) disclosed a multi-channel receiver for making phase measurements that allows a range measurement to be made. More recently, Sullivan (U.S. Pat. No. 5,999,131) disclosed a network of receivers isolating the direct path signal from a transmitter. Relative phase difference measurements between receivers in the network are converted into differential range estimates for locating the transmitter. Sullivan's system has the disadvantage of requiring a common time base or synchronization among all receivers in the network.
If a transmitter and a receiver are synchronized, then a precise phase measurement at a receiver can yield range information, up to a 360° phase uncertainty. In other words, a synchronized receiver can determine the location of a transmitter relative to the start and end of a wavelength, yet not be able to determine whether the transmitter's position lies within (for example) the seventh, or eighth or some other wavelength away. If the transmitter's absolute (or reference) position is determined initially by some other means, then the receiver can track the change in position of the transmitter relative to the established reference. Precise synchronization is essential to achieving meaningful range information in such a system. Any clock drift between the transmit-only target and the receiver results in a range error. As a practical matter, however, precise synchronization is exceedingly difficult and often expensive to achieve.
Transmit-only ranging may also be accomplished with an unsynchronized transmit-only target, using a network of synchronized receivers. The relative difference in received phase can be translated into a relative difference in position, subject to a 360° phase ambiguity.
All of these transmit-only ranging schemes rely on the “far-field” assumption: one must assume that a transmit-only target and a receiver are located at least a half wavelength apart. If a transmit-only target and a receiver are located within a half wavelength or less of each other, then near-field ambiguities make it difficult to determine an accurate range.
The simplicity of a transmit-only ranging system is attractive. However existing transmit-only ranging systems suffer from significant disadvantages. Some transmit-only ranging systems are dependent on precise synchronization of a network of receivers that tend to be complex, difficult, and expensive to implement. Some transmit-only ranging systems are dependent on measurement of a precise time between transmission and reception in order to calculate distance by multiplying time and signal velocity. Some transmit-only ranging systems are dependent on a similarly difficult synchronization of transmitter and receiver. Some transmit-only ranging systems are dependent on calibrating a transmitter to a known position before an absolute range can be determined. Some transmit-only ranging systems are dependent on a predictable variation between range and amplitude seldom found in the real world.
As far as the inventors are aware, prior art electromagnetic tracking and ranging systems are dependent on far-fields: radiated electromagnetic fields received at distances on the order of a wavelength or (usually) much further. Even inventors such as Ranger (U.S. Pat. No. 1,639,667) who disclose operation at ranges on the order of a wavelength or less implicitly assume far-field signal behavior. No prior art ranging system known to the inventors exploits near-field signal phenomena in performing ranging or distance measurement. The present invention employs near-field signal phenomena to advantage and has none of the dependencies and shortcomings noted in prior art ranging systems.
Historical Context
Some of the earliest wireless communication systems involved near-field or inductive coupling. One example involved coupling telegraph signals between a moving train and an adjacent telegraph line. With the discoveries of Hertz (as put into practice by such innovators as Marconi, Lodge, and Tesla), the overwhelming emphasis of RF development focused on long range, far-field systems. Frequencies were relatively low by contemporary standards. The earliest development was in the low frequency (LF) band (30 kHz-300 kHz), and soon progressed to the medium frequency (MF) band (300 kHz-3 MHz), with some pioneering work extending into the high frequency (HF) band (3-30 MHz). This work was oriented toward the empirical. Engineers focused on practical techniques for radiating and receiving signals. Little work was done to define or understand the fundamental physics that enables the radio frequency (RF) arts. For example, in 1932 the eminent RF expert, Frederick Terman, could say, “An understanding of the mechanism by which energy is radiated from a circuit and the derivation of equations for expressing this radiation quantitatively involve conceptions which are unfamiliar to the ordinary engineer.” [Radio Engineering, First Edition, by Frederick Emmons Terman; McGraw Hill, Book Co., Inc., New York; 1932; p. 494.] At that time the frontier of the RF arts had just begun to probe the lower end of the very high frequency (VHF) band (30 MHz-300 MHz). One textbook from the period provides a spectrum chart that ends with “30,000 kHz-60,000 kHz: Experimental and Amateur; >60,000 kHz: Not Now Useful”. [Radio Physics Course, Second Edition, by Alfred A. Ghirardi; Farrar & Rinehart, Inc., New York; 1942; p. 330.]
Radio direction finding and ranging remained focused on long range, far-field applications such as radio navigation for airplanes and radio guidance systems. The Japanese homed in on a Honolulu radio station in their attack on Pearl Harbor [Joe Carr's Loop Antenna Handbook, First Edition, by Joseph J. Carr; Universal Radio Research, Reynoldsburg, Ohio; 1999; p. 85.] Only in the 1940's, with the development of RADAR, did a theoretical emphasis in the RF arts catch up with the longstanding empirical emphasis. By then however, the RF frontier had rapidly passed through VHF and UHF and moved on to microwaves. The LF, MF, and even the HF bands were increasingly a backwater far removed from the active attention of most RF engineers.
In short, by the time fundamental electromagnetic theory began to be actively applied by RF engineers, RF engineers were not actively focused on applying this theory to the problem of radio ranging at low frequencies such as those in the LF, MF, and HF band. By and large, the overwhelming emphasis in the RF arts has been toward far-field systems, ones that operate at ranges beyond a wavelength, rather than near-field systems that operate at ranges within a wavelength or so.
Lower frequencies have certain advantages over higher frequencies. Lower frequencies tend to diffract better around obstructions and thus can be used in non-line-of-sight applications such as over a hill or around a building. Because of the longer wavelengths associated with lower frequencies, multipath interference is far less of a problem than at higher frequencies. Further, lower frequencies tend to be more penetrating of foliage and typical building materials, such as wood, brick, or concrete. Lower frequency RF circuits tend to be easier to build, and more robust. Components for use at lower RF frequencies tend to be less expensive and more readily available than those for use at higher frequencies.
Operation in the near-field, at ranges within a wavelength or so, yields certain advantages as well. Near-field signal levels tend to be far higher than would be predicted from the usual inverse range square   (      1          r      2        )far-field radiation relationships. In contrast, signal levels in the near-field decrease more rapidly than in the far-field, decreasing in intensity as a function of       1          r      4        .As a result, there is a lesser problem with electromagnetic interference among adjacent near-field systems so that it is easier to re-use the same frequency in a smaller cell size than would be expected from the usual far-field predictions. In short, electromagnetic waves behave differently in the near-field than in the far-field, and the inventors have discovered that the continuous and predictable variation of certain electromagnetic parameters may be used as signals traverse the near-field en route to the far-field to ascertain range or distance information.
Despite these near-field advantages, to the best of the knowledge of the inventors, no prior art describes a system in which near-field signal phenomena and the predictable behavior of those phenomena as they transition from near-field to far-field behavior are exploited in order to obtain range or distance information.
There is a need for an electromagnetic ranging apparatus and method that can be operated asynchronously without requiring synchronization of transmitter to receiver or synchronization among a network of receivers.
There is a further need for an electromagnetic ranging apparatus and method that can be operated without an awkward and lengthy calibration process and that can be useful in a wide variety of propagation environments.
There is another need for an electromagnetic ranging apparatus and method that can be used as part of a location or position tracking system.
There is yet a further need for a system and method for finding the range to or position of a source of electromagnetic signals whose location is unknown.
There is still a further need for a system and method of electromagnetic ranging that operates using relatively low frequencies and takes advantage of the characteristics of near-fields.